In practice, the training mileage of most runners is determined by personal preference as much as by physiological factors. Most runners run less than they could and would run better if they ran more, but they prefer not to run more, generally because they deem the small performance boost they might get from additional running to be not worth the additional time and suffering. Thus, in the real world, a runner's habitual mileage says as much about his or her level of seriousness as it does about whether that training load is optimal. In fact, this practice of self-limiting is so common that it never crosses the minds of many runners to train more than they do. In Chapter 9, which deals with injuries, Table 9.
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1 (page 192) presents typical running mileages for runners of different levels of seriousness. Its purpose is to show you the minimum amount of running you should expect to be able to do without an unacceptable frequency of injury. The table is relevant to the present context too, but I chose not to place it here because I want to encourage you not to place an artificially low ceiling on your running mileage, as I believe many runners do, because of the existing norms. There is nothing wrong with deciding you are not serious enough about running to really push the limit of your mileage tolerance, but I don't want to see you run less just because nobody ever suggested, explicitly or implicitly, that you could run more. Doubling in Running.